This invention generally relates to an improved board-level electromagnetic interference (EMI) shield for applications where heat dissipation is important. More particularly, the present invention relates to a shield that is easily removable, compatible with single or multi-compartment shield designs, thin in profile, lightweight, and promotes enhanced thermal dissipation of the shielded components. This solution is particularly advantageous for use in small electronic devices, such as cellular phones, personal digital assistants, handheld and laptop computers, among others.
EMI shields limit electromagnetic radiation from entering or exiting sections of a printed circuit board (PCB) containing electrical components. An EMI shield is further defined to be xe2x80x9cboard-leverxe2x80x9d if it attaches directly to the surface of a PCB, and without external or loose fasteners (such as screws or bolts), thereby eliminating a significant source of size and weight from the shielding scheme. Also as a result of this direct surface attachment to the PCB, board-level shields allow for EMI isolation of one or more sections of a PCB surface from one or more sections of the same (or other PCB""s) within a given device. As with other (non-board-level) shielding schemes, board-level shields also allow for EMI isolation of one or more sections of a PCB surface on a device, from other external devices.
Often, electrical components that need to be shielded also generate heat while operating, such as power amplifiers, central processing units (CPU""s), graphics chips, etc. Since the performance of these components often degrades at elevated temperatures, it is typically desirable to facilitate the transfer of heat away from them. This is typically done through the use of conduction, by utilizing a thermally conductive interface (TCI) material and a heat sink. The TCI material fills a gap in the stack-up between the component and the heat sink, and has a thermal conductivity higher than that of air, and typically as high as possible. TCI materials are often made of a conformable elastomer (such as silicone), filled with highly conductive thermal particles. The heat sink can be a finned metal block, a spreader plate comprised of sheet metal or plastic, a heat-pipe assembly, or any structure that enhances the dissipation of heat away from the electrical component and TCI material.
A common type of board-level EMI shield is known as a xe2x80x9ccanxe2x80x9d. A can is soldered to the ground trace on a PCB, directly over the electrical components that need to be shielded. Such cans offer extremely high levels of shielding effectiveness, are typically very reliable, and are well known in the industry. They are often installed in a fully automated fashion via a surface mount technology (SMT) process at the same time the components themselves are installed onto the PCB, using solder paste and a reflow process. Cans are extremely difficult to remove and replace (if the shielded components need to be reworked), because of the complicated de-soldering and re-soldering process.
Often, the components shielded by cans generate significant heat. Cans, however, require mechanical clearance (i.e. an air gap) above the components they are shielding on a PCB, to allow for proper soldering of the can to the ground trace during the reflow process. Because of this necessary clearance, a TCI material cannot be used to fill this air gap, which negatively impacts the transfer of heat away from the component to be shielded. This makes cans an undesirable solution where thermal dissipation is required.
A novel board-level shield is disclosed in U.S. patent application Ser. No. 09/793,754, of Reis, which describes a removable, multi-cavity shield, that utilizes a plurality of discrete electrically conductive fastening units, (such as BGA solder spheres) as a removable attachment mechanism. This reference, however, does not contemplate the need for improved thermal dissipation of components that are being shielded.
A technique for combining EMI shielding and thermal dissipation is disclosed in U.S. Pat. No. 6,347,035. While recognizing the need to solve both of these problems, the reference teaches only how to accomplish shielding of an entire PCB (e.g. motherboard), which necessarily includes both sides. That is, since the shielding enclosure described is not board-level, the shield cannot isolate sections of the same PCB (on the same side or opposite sides) from each other. References 5,436,803 and 5,597,979 teach similar, bag-like shielding schemes that do not shield at a board-level, and which additionally, do not contemplate the need for thermal dissipation.
Reference 5,175,613 discloses a package that combines EMI, ESD, thermal and mechanical shock protection of circuit chips. Also, reference 6,122,167 discloses an integrated hybrid cooling device with EMI shielding for a portable computer. The shielding schemes in both of these references are not board-level, however, since they do not allow for shielding sections of the same PCB from each other (e.g. one chip from another). Additionally, the solutions require an attachment mechanism involving screws or bolts, and holes that pass through the PCB. This consumes valuable PCB space, as well as adding significant weight to the design.
What has not heretofore been provided, and what is needed, is a board-level shield that is simultaneously removable, compatible with single or multi-compartment designs, thin in profile, lightweight, and allows for thermal dissipation of shielded components.
This invention provides an apparatus having a substrate having at least one electrical component disposed thereon; a plurality of discrete electrically conductive fastening units disposed in a pattern on the substrate surrounding the at least one electrical component; a board-level electromagnetic interference (EMI) shield comprising an electrically conductive layer; a plurality of apertures formed in the board-level EMI shield such that the apertures correspond to the pattern of the electrically conductive fastening units; with at least one thermally conductive interface (TCI) material disposed over the at least one electrical component; and wherein the electrically conductive layer of the board-level EMI shield is in electrical contact with at least one electrically conductive fastening unit.
In another aspect, this invention provides a substrate having at least one electrical component disposed thereon; a plurality of solder spheres disposed on the substrate surrounding the at least one electrical component; a board-level EMI shield comprising at least one compartment adapted to cover the at least one electrical component, the EMI shield further comprising an electrically conductive layer; at least one TCI material disposed over the at least one electrical component; a heat sink disposed over the at least one TCI material; wherein the electrically conductive layer of the board-level EMI shield is in electrical contact with at least one of the solder spheres, and wherein the board-level EMI shield and the solder spheres combine to limit electromagnetic radiation from entering or exiting the at least one compartment; and wherein the heat sink and the at least one TCI material combine to dissipate heat from the at least one electrical component.
In another aspect, this invention provides a board level EMI shield for a substrate having at least one electrical component disposed thereon and a plurality of discrete electrically conductive fastening units disposed in a pattern on the substrate surrounding the at least one electronic component, the board-level EMI shield comprising an electrically conductive layer; a plurality of apertures formed in the board-level EMI shield such that the apertures correspond to the pattern of the electrically conductive fastening units; at least one TCI material attached to the board-level EMI shield; wherein at least one of the apertures has a contact region and wherein the electrically conductive layer of the board-level EMI shield at the contact region of the aperture are deflectable to the extent necessary to allow the contact region to engage and retain at least one of the electrically conductive fastening units; and wherein the electrically conductive layer of the EMI shield at the contact region is adapted to make electrical contact with at least one electrically conductive fastening unit.